GPS World, August 2015

Space Mission REMOTE SENSING THE RED ELLIPSES SHOW the MMS orbit paths during the first and second phases of the mission Each spacecraft uses GPS signals that come from satellites situated along the green circle shown surrounding Earth to track its position Credit NASA MMS Navigator Highlights At the highest point of the MMS orbit at more than 43500 mile above the surface of the earth Navigator set a record for the highest ever reception of signals and onboard navigation solutions by an operational GPS receiver in space At the lowest point of the MMS orbit Navigator set a record as the fastest operational GPS receiver in space at velocities over 22000 miles per hour At the farthest point in its orbit some 43500 miles away from Earth Navigator can determine the position of each spacecraft with an uncertainty of better than 50 feet downlink telemetry and scientific data to calculating when ground controllers would command the firing of the satellites onboard thrusters which move and help maintain their orbital formation an exercise that will happen at least once every couple weeks I think theres a good chance well end up being able to use GPS and save us some of the expense of using ground observations Robertson said While Navigator technology and GPS receivers were previously flown for testing and to help navigate a lowearth orbit mission this is the first time that the complete Navigator package has been used to actively navigate a high altitude mission Now that the team knows it works so well Navigator can be used for other missions that travel in similar high orbits THE FOUR MMS OBSERVATORIES are processed for launch in a clean room at the Astrotech Space Operations facility in Titusville Fla The MMS mission launched March 12 2015 Credit Ben Smegelsky NASA assuring continuous contact with the tracked GPS satellites Spinning adds a whole new dimension to trying to figure out where you are said Ken McCaughey MMS GPS Navigator Product Development Lead at Goddard As the spacecraft rotates we have an algorithm running that allows us to hand off from one antenna to the next without losing the signal Robust Receivers To the satisfaction of the technologys architect Goddard technologist Luke Winternitz the receivers have proven very robust Shortly after the GPS receivers were powered on after the launch Navigator became at more than 43500 miles above Earths surface the highest ever operational GPS receiver in space Were tracking up to 12 GPS satellites at maximum altitude and track on average about nine Winternitz said Were really excited about their performance so far Even if the receiver were to lose all GPS signals for part of the orbit Navigator is specifically designed to handle such dropouts By gathering as many observations as possible integrated software called GEONS Goddard Enhanced Onboard Navigation System can still compute the orbit by incorporating additional information including drag force gravity and solar radiation pressure This system will be even more important during the second phase of the MMS mission when the orbit will double in size and travel all the way out to 95000 miles from Earth Its going to be very interesting to see how far out MMS can still receive signals said Mission Deputy Project Manager Brent Robertson But Navigator has already far exceeded expectations Almost all activities associated with operating the mission depend on where the satellites will be positioned a few days hence That includes everything from determining the best time to www gpsworld com August 2015 GPS World 51

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